Devices for incorporating filters for filtering fluid samples

ABSTRACT

Devices and use thereof, the device comprising a filter and a means for region specific compression of the filter. Alteratively, the device comprises a filter; a region containing the filter; a fluid access port to the region containing the filter; a fluid egress port from the region containing the filter; and a lateral fluid flow path through the filter connecting the fluid access port to the fluid egress port. Alternatively, a single step assay device whereby fluid movement through the device occurs substantially solely due to action of capillary force, the device comprising a filter; a region containing the filter; a fluid access port to the region containing the filter; a means for retarding movement of particles through a peripheral filter surface; a fluid egress port from the region containing the filter; a lateral fluid flow path through the filter connecting the fluid access port to the fluid egress port, whereby sample fluid substantially devoid of particulate matter is released from the filter through the egress port; and, an exit region fluidly connected to the egress port. Use of the device optionally comprises means for a producing an assay result in an exit region of the device.

FIELD OF THE INVENTION

This invention relates to devices comprising filters for filtering fluidsamples. In a particular embodiment, the filter-containing devicesfilter cells or particulate matter from biological samples and introducethe filtrate into a capillary space, and operate without the use of anyexternally applied force.

BACKGROUND ART

With the advent of point of care testing in hospital emergencydepartments, it has become increasingly important to develop diagnosticproducts which are simple, rapid and convenient for the user to perform.This need has arisen because health care workers in an emergencydepartment need results rapidly with a minimum of time given to theperformance of a diagnostic test. Providing a diagnostic result inminutes allows a physician to treat a patient as soon as possible.

Point of care diagnostic tests frequently are performed on biologicalsamples, such as whole blood or urine. Cells and particulate matter inbiological samples can interfere with fluid flow in a test device, andthus impair the measurement of analytes in the biological fluid.

For example, in blood, red blood cells can interfere with spectroscopicmeasurements, and as the hematocrit varies, the volume of plasma in agiven volume of blood varies. To overcome these problems, red bloodcells are separated from plasma to allow for a more defined and uniformsample.

As a further example, urine can contain lymphocytes that can affectspectroscopic measurements, and flow through filters and capillaries.Thus, a device to filter out cells, particulate matter, or debris from abiological sample can improve the quality of an analytical procedureperformed on the sample.

To achieve removal of particulates, incorporation of a filter into anassay device has been described in the prior art. For example, U.S. Pat.Nos. 4,477,575; 4,753,776; 4,987,085; and 5,135,719 refer to bloodfilter devices whereby a transverse flow of blood through a filterresults in the separation of red blood cells from plasma. Sealing of thefilter in the device to achieve effective filtration, and not allowsample to bypass the filter, has been a problem in the prior art. Asmall capillary or gap between the filter and the filter chamber wallsoften existed due to poor initial sealing, or because the gap formedwith time. As a consequence, particulates in a fluid sample travel inthe capillary space or gap rather than through the filter. Particulatematter which travels around the filter decreases the filtrationefficiency, repeatability, and may cause the filter to be unacceptablefor certain applications. Techniques, such as using glues, tapes and thelike have been used to seal a filter into the filter chamber of suchdevices. The use of these materials to affect sealing has producedvariable, and often poor sealing. Additionally, these sealing methodsresulted in absorption of variable amounts of the sealing compound intothe filter.

Another drawback of prior art filter devices is consequent to use of arelatively short transverse fluid flow path through a filter. Thetransverse flow path in a conventionally shaped filter (a filter with alength, width and substantially thinner depth) is the distance betweenthe top and the bottom of the filter, the filter depth commonly referredto as the filter thickness Filters are generally 0.1 mm to 6 mm thick,this relatively short flow path produces relatively poor separationefficiency. A longer flow path would allow more particulates to beremoved from fluid, thus, increasing the separation efficiency. U.S.Pat. Nos. 4,678,757; 5,135,719; 5,262,067 and 5,435,970, comprisefilters treated with materials such as carbohydrates, agglutins andlectins, to affect separation of red blood cells from plasma. However,due to the relatively short fluid flow path, the filtration efficienciesin these teachings are not optimal. European Appl. No. 89300416.8,describes methods and devices which bind red blood cells to treatedpolycationic filters. However, treatment of filters introduces anadditional process in device fabrication. A filter-device design thatdoes not require treatment would be advantageous since the filter wouldbe too costly and device manufacture would be less complicated; lesscomplicated device designs are easier and more cost-effective tomanufacture.

Embodiments with longer transverse flow paths have also beendisadvantageous, however. U.S. Pat. No. 5,139,685 (“the '685 patent”),describes a cylinder of stacked filters, so that the device has arelatively long flow path. Although the '685 patent has a relativelylong transverse fluid flow path pursuant to stacking of discretefilters, applications of this technique are limited. Namely, in the '685patent, discrete filters are stacked and are under an applied pressureto achieve an efficient filtration of red blood cells from plasma. Thepressurization of the filters is necessary to achieve a fast andefficient separation of particulate matter from the sample.

The relatively large amount of space required and the configuration of adesign of the '685 patent does not lend itself to a convenient point ofcare diagnostic testing. Point of care diagnostic testing is facilitatedby smaller and more convenient designs that can be easily manipulated bya health care worker, designs which are capable of being fed intohand-held instruments that provide quantitation of assay results.Devices capable of being fed into hand-held instruments (such as areader) should be small and flat, and have smooth surfaces. Preferably apoint of care device would not require an externally applied pressure.

Thus, there is a need for an efficient, compact, cost-effectivefiltration device. There is also a need for a means to effectively seala filter within a device, whereby the fluid flow path is optimized,leading to increased filtration efficiency. Most desirably, there is aneed for a sealing means that makes device fabrication tolerances lesscrucial and device manufacture more economical.

DESCRIPTION OF FIGURES

FIGS. 1A, 1B, 1C, and 1D show one embodiment of a device in which FIG.1A is a top view of base 10. FIG. 1B is a cross-section of base 10. FIG.1C is a top view of an assembled device taken along plane 1B-1B in FIG.1A. FIG. 1D is a cross section of an assembled device including a filter20, lid 18, and base 10 taken along plane 1D-1D in FIG. 1C.

FIGS. 2A, 2B, and 2C show another embodiment of a device in which FIG.2A is a top view of base 10. FIG. 2B is a cross-section of base 10 takenalong plane 2B-2B in FIG. 2A. FIG. 2C is a cross section taken along thecross-sectional plane in FIG. 2B of an assembled device including afilter 20, lid 18, and base 10.

FIGS. 3A, 3B, and 3C show another embodiment of a device in which FIG.3A is a top view of base 10. FIG. 3B is a cross-section of base 10 takenalong plane 3B-3B in FIG. 3A. FIG. 3C is a cross section taken along thecross-sectional plane in FIG. 3B of an assembled device including afilter 20, lid 18, and base 10.

FIGS. 4A, 4B, 4C, 4D and 4E show another embodiment of a device in whichFIG. 4A is a top view of base 10. FIG. 4B is a cross-section of base 10viewed along the plane 4B-4B of FIG. 4A. FIG. 4C is a cross section,along the same plane as FIG. 4B, of an assembled device including afilter 20, lid 18, and base 10. FIG. 4D is a top view of an embodimentcomprising a lid cavity 42, depicted in dashed lines. FIG. 4E is a crosssection of FIG. 4D taken along the plane 4E-4E.

FIGS. 5A, 5B, and 5C show another embodiment of a device in which FIG.5A is a top view of base 10. FIG. 5B is a cross-section of base 10viewed along the plane 5B-5B of FIG. 5A. FIG. 5C is a cross section,along the same plane as 5B, of an assembled device including a filter20, lid 18, and base 10.

FIGS. 6A, 6B, 6C and 6D show another embodiment of a device in whichFIG. 6A is a top view of base 10. FIG. 6B is a cross-section of base 10,viewed along plane 6B-6B of FIG. 6A. FIG. 6C is a top view of anassembled device of this embodiment. FIG. 6D is a cross section, viewedalong the plane as 6D-6D of FIG. 6C, of an assembled device including afilter 20, lid 18, and base 10.

DISCLOSURE OF THE INVENTION

The present invention provides for simple and rapid filtering ofbiological samples, whereby a sample can be analyzed in the same deviceor a different device.

In one embodiment, the present disclosure teaches the use of lateralflow through filters and the use of capillary force to cause the exit offiltrate fluid from filters and into a capillary space.

Alternative embodiments of a device of the invention comprise are wherethe device comprises a filter and a means for region specificcompression of the filter. Alteratively, the device comprises a filter;a region containing the filter; a fluid access port to the regioncontaining the filter; a fluid egress port from the region containingthe filter; and a lateral fluid flow path through the filter connectingthe fluid access port to the fluid egress port. Alternatively, a singlestep assay device whereby fluid movement through the device occurssubstantially solely due to action of capillary force, the devicecomprising a filter; a region containing the filter; a fluid access portto the region containing the filter; a means for retarding movement ofparticles through a peripheral filter surface; a fluid egress port fromthe region containing the filter; a lateral fluid flow path through thefilter connecting the fluid access port to the fluid egress port,whereby sample fluid substantially devoid of particulate matter isreleased from the filter through the egress port; and, an exit regionfluidly connected to the egress port. Use of the device optionallycomprises means for a producing an assay result in an exit region of thedevice.

LIST OF REFERENCE NUMERALS 10 Base 12 Filter Cavity 14 Fluid Access Port15 Fluid Access Region of Filter 20 16 Fluid Egress Region of Filter 2017 Exit Region 18 Lid 20 Filter/Filter Matrix/Membrane 22 CompressionStructure 23 Support Bar 24 Dead Space of Filter Cavity 12 26 SampleReservoir 28 Filter Supports/posts 30 Vent Holes 32 Filter Stays 34Grooves 36 Lateral Compression Region of Compression Structure 22 38Proximal Compression Region of Compression Structure 22 40 DistalCompression Region of Compression Structure 22 42 Lid Cavity

MODES FOR CARRYING OUT INVENTION

This invention describes novel devices comprising filters for the rapidfiltering of samples, particularly chemical, environmental, orbiological samples, and preferably for introduction of the filteredsample into a capillary space. This invention can be utilized in anydevice format in accordance with the teachings provided herein. Forexample, although assay device filters of conventional configuration arediscussed, it is understood that the principles of the application applyto devices with other configurations. In a preferred embodiment, theinvention is used with the technology of devices described in U.S. Pat.No. 5,458,852 to Buechler, incorporated herein by reference. Aspects ofthe invention are discussed below.

Lateral and Transverse Fluid Flow

As used herein, fluid flow will be described as follows: a conventionalfilter has a width, length, dimensions and substantially smaller depth(“thickness”) dimension. For a filter with such dimensions, transverseflow is perpendicular to the length and width of the filter, and ispredominantly in a direction parallel to the depth of the filter.Conversely, lateral flow is predominantly parallel to the length orwidth planes of a filter. Alternatively, a lateral flow path is adistance greater than a transverse flow path distance through a filter,typically these flow paths are oriented perpendicular to each other. Inone embodiment of the invention, a lateral fluid flow path is through afilter connecting a fluid access port to a fluid egress port, where theflow path is greater than or equal to that of the greatest crosssectional distance of the filter as determined perpendicular to anypoint along the flow path.

Conventional device filters comprise some degree of capillarity. To acertain extent, fluid moves through the filter due to capillary forcescreated, e.g., by small diameter pores or the close proximity of fibers.Fluid may move through such a filter with or without external pressure.On a micro scale relative to the filter's overall dimensions, fluidtravels in multiple directions as a result of capillary forces. On amacro scale, however, fluid travels in one direction; i.e., through thefilter in a predominantly lateral or transverse direction to a locationwhere fluid can exit the filter.

To filter a sample, either lateral or transverse fluid flow requires atleast one fluid input surface and at least one fluid egress surface. Afluid input surface is defined as the filter surface where unfilteredsample fluid is placed in contact with the filter; the fluid egresssurface is a filter surface from which the majority of a filtered samplefluid (“filtrate” or “filtration liquid”) exits. As discussed below, asmaller amount of fluid can exit from peripheral surfaces-in certainembodiments. A peripheral surface is defined as being a surface that isnot a fluid input surface and not a fluid egress surface. For transverseflow, the input and egress surfaces are the top and bottom surfaces of aconventionally shaped filter, e.g., surfaces parallel to the length andwidth planes. Alternatively, for a filter having lateral fluid flow,input surfaces and egress surfaces can be on the top, bottom or anysurface which is not a peripheral filter surface.

In accordance with the present disclosure, a preferred flowdirectionality through a device filter is lateral flow. Compared totransverse flow, lateral flow has several advantages. First, the fluidflow path can be dramatically increased in a filter of a standardconfiguration. For a filter of conventional shape, lateral flow yieldsgreater filtration efficiency than transverse flow since the lateralfluid flow path is longer than the transverse fluid flow path. Forexample, conventional assay filters generally have a thickness ofbetween 0.1 mm and 6 mm, approximately 2 mm is most common, with alength and width substantially longer than the thickness usually on theorder of several centimeters. Thus, lateral flow yielded greaterfiltration efficiency with minimum effects on the device's shape.Second, lateral flow within a filter allowed the addition of fluid toany region of the filter that is not a fluid egress region, so long asfluid could laterally flow through the filter to the fluid egress regionthereof. In lateral flow devices, addition of fluid to regions of thefilter which are not fluid egress regions allowed fluid to enter thefilter at an effectively larger area (approx. 2 times) than by enteringonly on one side. The larger area for fluid entrance provided a moreefficient use of the filter, maximizing flow rate through the filter andminimizing the potential for clogging with particulate matter which isto be filtered from a sample.

Additionally, incorporating a single filter rather than a stack ofseveral filters in a device conserves device space and permits thefilter to be easily situated on a device, allowing a plethora ofpossible design configurations.

Filter Materials

Various filter materials are available for filtering cells andparticulate matter from biological samples. For example, cellulosefibers, nylon, glass fibers, polyester fibers or combinations of thesematerials are useful to make filters that remove debris from samples,e.g., cells from urine and red blood cells from plasma.

The filter is preferably chosen such that the pores or fibrous matrix ofthe filter do not get clogged by particulate matter from the sample. Inthe case of separating red blood cells from plasma, filters generallybind, retain or retard the red blood cells and allow plasma to separatefrom blood and pass through the filter.

Filters can comprise a fibrous matrix or porous material. A preferredfilter is a fibrous matrix filter that is made of borosilicate glassmicrofibers. Borosilicate glass microfiber filters permit filtration ofwhole blood samples by permitting the sample (including particulatematter such as red blood cells) to penetrate the filter. For whole bloodsamples, the filter retards red blood cells and allows plasma to movethrough the fiber matrix at an nonretarded, higher flow rate. When thesefilters were used, and cells and plasma moved through the filter, theplasma moved further ahead of the red blood cells.

Alternative filters can also be used which have a more porous structure,for example filters comprising nitrocellulose, acrylic copolymers, orpolyethersulfone. These filters, generally, functioned differently thanfilters comprising fiber matrices, in view of the fact that they havepore-like passages typically of uniform diameters. Typically, a porousfilter is selected such that the pore diameters are smaller than thediameters of the particulates desired to be separated from a sample.These filters retain rather than retard the red blood cells, i.e., redcells generally do not penetrate the filter beyond its surface. Oneskilled in the art will recognize that if the particulate matter in asample is approximately the same size or greater than the pores of afilter, the particulate matter will rapidly clog the pores and slow orstop fluid flow therethrough.

Thus, in accordance with the present disclosure, various filters can beused. A filter may be one of several commercially available filtersincluding but not limited to Ahlstrom CytoSep (Mt. Holly Springs, Pa.)or Micro Filtration System's (Dublin, Calif.) glass fiber media. U.S.Pat. Nos. 5,186,843; 4,774,039; 4,629,563, and 5,171,445, cover thecompositions of these and like media.

Sample Reservoir

Preferably, a sample reservoir accomplishes several functions: 1) itdelimits a volume which is sufficient to achieve an assay result, andthus facilitates a device user's ability to provide the suitable volume;2) it accomplishes the foregoing while allowing for a diverse range ofinput volumes in a manner which does not impair an assay result; and, 3)a reservoir that is a capillary space helps to prevent fluid escape. Ina preferred embodiment, the sample reservoir contained a fluid volumeapproximately 100 times the volume needed for downstream processing of asample.

As depicted in FIGS. 6A, 6B, 6C and 6D, a sample reservoir 26 comprisinga capillary space is incorporated into the device, whereby sample wascontained in the capillary space of the sample reservoir, and thereservoir was in fluid communication with an edge, top and/or bottom ofthe filter (FIG. 6D). An advantage of utilizing a capillary space forintroducing the sample to the filter is that the fluid, for example, ahazardous chemical, environmental or biological fluid, contained in acapillary space will tend not to spill or leak from the device.

“Sealing” of Peripheral Surfaces of a Filter

Preferably the peripheral surfaces of the filter are sealed within thedevice: 1) so that fluid must flow through the filter; and 2) to preventthe fluid sample, e.g., particulates, from flowing around the filter ina capillary between the filter and the device, thus contaminating thefiltrate. Sample that travels around the filter in a capillary space,rather than through the filter, can enter the exit region 17. Whenunfiltered fluid enters exit region 17, the filtering efficiency of thedevice is decreased. “Sealing” of the peripheral surfaces need not beliquid-tight, sealing in accordance with the invention also comprisesthe ability to retard to flow of liquid and particulates, orparticulates into spaces between the filter and the walls of the deviceadjacent to the filter.

There are several methods to achieve sealing of the peripheral surfacesof a filter. Preferably the sealing is liquid-tight, although someliquid release is acceptable so long as a low resistance fluid flow pathoutside the filter does not result. In one embodiment, the sealingpermits liquid between the device wall and the filter, but retards thedevelopment of a capillary space along the peripheral surfaces and thesealing also may serve to retain particulate matter in the filter andnot in any space along peripheral surfaces, due to compression. In anyevent, there is preferably particle-tight, more preferably liquid-tightand most preferably fluid-tight sealing of the device at or near thefluid access region 15 of filter 20.

Regardless of the flow path direction, filter surfaces may be sealedusing one or a combination of the following techniques:pressure-adhesive tapes, glues, or sealants. Such techniques requirecareful placement of the sealant along edges of the filter and the sidewalls of filter cavity 12. A preferred liquid sealing of the peripheralsurfaces is achieved by a pressure fit. A pressure fit may be achievedby placing a slightly oversized filter into filter cavity 12 such thatall the peripheral surfaces of the filter are in contact with the wallsof the filter cavity. Concerning length and width dimensions, a filteris preferably 1-10% and more preferably 1 to 5% larger than the filtercavity into which it is to be placed to ensure direct contact.Compression of the depth dimension is discussed in greater detail below.Direct contact between the filter and the filter cavity wallsdiscourages fluid from traveling between the peripheral surfaces of thefilter and the filter cavity walls, because a capillary space does existbetween the filter and the device walls. Thus, the perimeter or sideedges of the filter will maintain contact with the walls of the filtercavity 12 as depicted in FIG. 1A.

The filter must have a degree of resilience such that it may be squeezedand hold its shape over time. The conventional media described hereinhave these properties and can be employed as disclosed herein toeffectively seal particulates and/or liquid. The more pliable a filteris, the larger the filter must be relative to the cavity into which itis placed, so as to assure that the filter provides pressure against thecavity walls and a low resistance fluid flow path is avoided.

To prepare a device in accordance with the invention, the shape of thefilter must be approximately identical to the filter cavity, 12,(FIG.1A). Filter cavity 12 may be of any three dimensional shape, e.g.,trapezoidal, rectangular, rounded or the like. A presently preferredfilter shape is that of the filter cavity 12 of FIG. 1A-1D.

Region Specific Compression of the Filter

Preferred embodiments of the invention make use of compression of thefilter in distinct areas to limit or retard the movement of particulatematter (e.g., cells or debris) within the filter and to preventparticulate matter from escaping the filter and traveling along theperipheral filter surfaces. In the case of preventing fluid from flowingalong peripheral surfaces of the filter, a compression structure 22(e.g., see FIG. 2, 3, 4 or 5A) in filter cavity 12 helps to prevent theformation of capillary gaps between the filter 20 and a surface to whichit is in contact, e.g., lid 18 or a surface defining cavity 12.Capillary gaps are to be avoided as they provide a low resistance flowpath to fluid exit region 17, and thereby permit unfiltered sample tocontaminate the filtrate.

In addition to region specific compression, liquid sealing of the filterperipheral surfaces by use of capillary force, glues, pressure-sensitivetapes, or sealants can be used to prevent particulate escape and flow ofliquid along the peripheral surfaces. It is advantageous to avoid glues,pressure sensitive tapes or sealants since they have a tendency to losetheir sealing properties with time; and the sealant in such materialstends to leach through the filters, potentially affecting the filtrate.In contrast, region specific filter compression leads to a lasting sealand functions without any additives or additional parts; thus, itdecreases the complexity of the fabrication process and avoids potentialcontamination of the filtrate.

Structures using region specific compression are shown, for example, inthe embodiment depicted in FIG. 4. Referring to the top view FIG. 4A, acompression structure 22 is shown. Preferred degrees of compression ofthe thickness of filter 20 by compression structure 22 and an abuttingsurface such as lid 18, are 1 to 50% and more preferably 1 to 30% of thenative thickness of the filter. Accordingly, in preferred embodiments ofthe device only specific regions of a filter are compressed; all regionsof the filter are not compressed, and flow rate is not unduly impeded.Thus, one advantage of the present invention is that effective and rapidfiltration is achieved.

The cross section view of an assemble device in FIG. 4C displays apreferred height change between filter cavity 12, and compressionstructure 22. This height change results in dead space 24 (e.g., in FIG.4C) underneath filter 20 upon placement of filter 20 in cavity 12.

Dead space 24 effectively leads to liquid sealing, since dead space 24has minimal capillary force, and therefore does not draw any fluid awayfrom or out of filter 20. The capillary force is low for dead space 24due to a relatively large gap between filter 20 and filter cavity 12.

Region specific compression leads to liquid sealing yet also facilitatesrapid fluid flow through the filter. The filter region depicted abovethe dead space is not compressed to the same degree as the filterregions between compression structure 22 and an abutting surface. Thepores or matrices of the filter are not appreciably compressed in thisarea and fluid flow therethrough is facilitated. It is advantageous tokeep the filter noncompressed in embodiments where maximum flow ratefrom the fluid egress region is desired.

In summary, dead space 24 results from certain compression structuredesigns and is useful for several reasons. First, it has minimalcapillary force and draws no fluid into its cavity. Thus, a liquid sealis formed along the respective filter surface adjacent the dead space,forcing fluid to flow through the filter. Second, the filter adjacent tothe dead space is not compressed and therefore the pore or fiberstructure within the filter remains unchanged; such filter configurationis in contrast to the prior art, and advantageously produces higher flowrates.

As noted, preferred degrees of compression of filter 20 by compressionstructure 22 and lid 18 are 1 to 50%, preferably 1 to 30% of the nativethickness of the filter. To achieve such compression, compressionstructure 22 need not be limited to one uniform level or height. Rather,compression structure 22 may be comprised of multiple sections at thesame or different heights. FIG. 5A displays an embodiment of the presentinvention in which compression structure 22 is comprised of sub-regionsincluding: support bar 23, lateral compression regions 36, proximalcompression region 38, distal compression region 40. Each sub-region maybe sized to yield compression that facilitates sealing, filtration, orflow rate properties. In the embodiment of FIG. 5, the combination ofsupport bar 23, lateral compression regions 36, and distal compressionregion 38 force filter 20 to be in contact with lid 18 forming apreferably liquid tight seal above each sub-region, thus facilitatingprevention of flow along the peripheral surfaces of the filter. Distalregion 40 does not form a liquid-tight seal, but permits liquid to exitat fluid egress region 16. Accordingly, filter 20 is not uniformlycompressed, but rather is compressed only above the sub-regions ofcompression structure 22.

Another advantageous aspect of region specific compression is theability to retard particulate matter from designated regions of thefilter. Namely, to retard particulate matter from reaching either theperipheral edges or the fluid egress region of a filter. Retardation ofparticulate movement occurs since compression of a filter causesconcomitant compression of the fiber spacing and/or pores in the filter;the compression of these microstructures makes it more difficult orimpossible for particulate matter or cells to travel therethrough.

One skilled in the art will recognize that the selective compression ofthe filter at the compression structure 22 is not required for theseparation of the particulate matter from the sample, but rather, suchcompression is but one embodiment than can be utilized for sealing theperipheral surfaces of the filter in a device and for modulatingparticulate movement. In a preferred embodiment, no particulate mattercan exit along the peripheral surfaces or the fluid egress region offilter 20. In an alternative embodiment employing compression structure22, particulate matter is retarded in its movement through the filter,but nevertheless is capable of exiting the filter; in this embodiment,the exit region preferably holds the amount of liquid that is requiredto produce an assay (via a material(s) or modality(s) for achieving anassay result appreciated by those of skill in the art), where thatvolume is less than the volume of fluid that flows through he filterahead of the retarded particulate matter.

Exit Region in Fluid Communication with Fluid Egress Region of Filter

In a preferred embodiment, after the particulate-free fluid has passedthrough the filter it is preferably drawn into a fluid exit region 17(see, e.g., FIG. 1D). Preferably, exit region 17 comprises highercapillarity than filter 20 to facilitate fluid flow therebetween. Thus,a preferred embodiment of the invention utilizes capillarity force,whereby fluid egress area 16 of the filter is immediately adjacent toexit region 17, and region 17 is a capillary space. Due to the use ofcapillarity as taught herein, fluid leaves the filter without anexternal pressure and uniformly fills the capillary space of exit region17, and fluid will not enter any region with lower capillarity than thefilter, e.g., dead space 24. Thus, capillary force can be used to causeegress of fluid from the filter and into an exit region 17 without theapplication of an external pressure such as hydrostatic pressure. Inaccordance with this embodiment, compression of the filter bycompression structure 22, in particular distal compression region 40,should not cause the pores of the filter to be made sufficiently smallsuch that the capillary force holding the fluid within the filter isgreater than the capillarity of exit region 17.

Device Assembly

Advantageously, devices that incorporate filters that function bylateral flow, can be designed so that the overall device thickness isnot constrained by the filter thickness.

The filtration devices described herein generally require assembly andjoining of several parts. Lid 18 and base 10 can be fabricated fromconventional materials compatible with chemical, environmental orbiological fluids to be assayed, for example: a plastic material such asacrylic, polystyrene, polycarbonate, or like polymeric materials; aswell as silicon composites, such as silicon semiconductor chips; glass;or metal. In the case of plastic polymeric materials, lid 18 and base 10may be fabricated using thermal injection molding technology ormachining. In the case of fabricating the lid and base from siliconcomposites, micromachining and photolithographic techniques, commonlyused in the field of electronics, can be utilized to create chambers andcapillaries.

The base 10 and lid 18 are contacted together in order to form thephysical configuration desired to achieve a particular result.Ultrasonic welding, adhesives, physical interfitting and heat weldingare some of the methods that may be used to join base 10 and lid 18. Forexample, with embodiments comprising a lid of silicon composite orplastic, and a base of plastic or silicon composite, the base and lidcan be joined with adhesives.

In a preferred embodiment, the plastic surfaces of base 10, lid 18, orboth are made hydrophilic or “wettable”, whereby the contact anglebetween the sample fluid meniscus and the base 10 and lid 18 isdecreased. There are several ways to decrease the contact angleincluding but not limited to corona discharge, plasma treatment or thedrying down of various surfactants or proteins onto surfaces. Inaccordance with standard methodologies, exposing a plastic surface to acorona discharge or plasma gas results in the formation of functionalgroups on the surface. The surface chemistry as well as the degree ofhydrophobicity are thus modified and can be used for a variety ofapplications. For example, time gates, as described in U.S. Pat. No.5,458,852 to Buechler (entirely incorporated by reference herein), canbe incorporated at the fluid egress region to provide an incubation timefor the sample within the filter.

EXAMPLES

One skilled in the art will recognize, in view of the presentdisclosure, that the filter cavity can have many possible designconfigurations or embodiments. The filtration concepts disclosed hereincan be incorporated into a variety of devices that can be used invarious assays. The following examples demonstrate presently preferredembodiments and are not intended to limit the invention.

Example One

An embodiment of the present invention is shown in FIG. 1. Base 10 wasfabricated using injection molding technology and was made of whiteacrylic copolymer (Polysar NAS®30, Polysar, Inc., Madison, Conn.). Theoverall device length, width and thickness were 8.7, 3.5 and 0.26 cmrespectively. The surface of base 10 was made hydrophilic by a coronadischarge treatment.

In accordance with standard methodologies, filter cavity 12 (FIG. 1A,1B) was formed into base 10. The filter cavity had a bilaterallysymmetrical trapezoidal shape, with parallel sides 1.0 and 1.34 cm longand 0.72 cm apart. A filter 20, slightly oversized 1-30% preferably 1-5%relative to filter cavity 12, was fitted into the cavity (FIG. 1D). Thefilter (#GB100R, Micro Filtration Systems (“MFS”), Dublin, Calif.)consisted of borosilicate glass fibers; its thickness was 0.038 cm andhad an absorption volume of approximately 50 μl.

A clear plastic lid 18 (FIG. 1C, 1D) was coupled to base 10 byultrasonically welding using a Branson 941 AE welder (Branson Inc.,Danbury Conn.) set at 50 joules and 40 psi. Bonding lid 18 to base 10slightly compressed the filter 20. This compression created a seal thatserved to prevent fluid and particle flow around the peripheral edges offilter 20. Referring now to FIG. 1D, fluid access port 14 was locateddirectly over the filter. Lid 18 and base 10 formed a capillary space inexit region 17 adjacent to fluid egress region 16 of filter 20. Despitethe increase in capillarity by compressing filter 20, exit region 17comprised sufficient capillarity to draw filtrate fluid, e.g. plasma,out of filter 20. In this embodiment the exit region comprised acapillary space, where a cross section of exit region 17 immediatelyabutting fluid egress region of filter 20 had dimensions approximately25 μm by 1.0 cm. The volume encompassed by this exit region ispreferably a volume sufficient to permit enough fluid to flow throughthe device so that an effective assay result is obtained (via a materialor modality appreciated by one of ordinary skill in the art) after whichthe exit region is filled with fluid and flow stops; for such anembodiment an escape port is present (not illustrated) that permitsrelease of gaseous fluids but not liquid fluids.

A use of the embodiment depicted in FIG. 1 was as follows: fresh humanwhole blood (70 μl), that was drawn in a Vacutainer® Blood CollectionTube with acid citrate dextros (ACD), was added to fluid access port 14.Some of the blood was immediately absorbed into filter 20 and theremainder formed a small droplet covering fluid access port 14. As thefluid moved by lateral flow through filter 20, two distinct flow frontsformed. Since the filter retarded flow of particulates, the two flowfronts consisted of a clear plasma front preceding a dark front of redblood cells. Thus, the plasma front reached egress region 16 of thefilter before the red cells, and particulate depleted filtrate enteredexit region 17 before the red blood cell flow front reached the end ofthe filter.

Example Two

Another embodiment of this invention is depicted in FIG. 2. Base 10 isfabricated using injection molding technology and is made of whiteacrylic copolymer (Polysar NAS® 30, Polysar, Inc., Madison, Conn.). Theoverall device length, width arid thickness were 8.7, 3.5 and 0.26 cmrespectively. The surface of base 10 was made hydrophilic by a coronadischarge treatment.

Base 10 comprising compression structure 22 was formed by conventionalmethods. A slightly oversized filter 20 (FIG. 2C) was fitted overcompression region 22 causing dead space 24 to be defined (FIG. 2C). Thefilter was 1-30%, preferably 1-5% larger than the portion of the filtercavity 12 which did not include the dead spare 24. The filter (MFSfilter, cat. no. GB100R Micro Filtration Systems, Dublin, Calif.)consisted of borosilicate glass fibers. Its thickness was 0.038 cm andhad an absorption volume of approximately 50 μl. Thus, there was sealingbetween fluid access port 14 and fluid access region 15 of filter 20, sothat essentially the only way sample could enter the filter was throughregion 15.

Advantageously, this embodiment comprises dead space 24 (FIG. 2C). Inprior art devices, fluid could accumulate in a capillary space betweenthe filter and device walls around the filter. The formation of fluid inthe capillary space is believed to result from a relatively highcapillary force created by the close contact of the filter with thefilter cavity and by the deformability of the filter. Fluid formation insuch regions is undesirable since it can provide a route whereby sampledoes flow entirely within the filter, and if particulates enter thespace separation efficiency is decreased. The present inventiondecreases potential fluid formation between filter 20 and a wall ofcavity 12 by means of a space (dead space 24) that has virtually nocapillary force. Therefore, so long as capillary force is used to movefluid through the filter fluid exits only from the designated region,namely, fluid egress region 16.

A clear plastic lid 18 (FIG. 2C) was coupled to the base by ultrasonicwelding using a Branson 941 AE welder set at 50 joules and 40 psi.Bonding of lid 18 to base 10 slightly compressed filter 20 abovecompression structure 22: the filter was compressed by 1-50%, preferably1-30% relative to its native thickness. This region specific compressioncreated a sufficient seal to serve to prevent fluid flow over theperipheral surfaces of the filter at that region and to retard particleswithin the filter compressed at that region. Fluid access port 14 waslocated directly over the trapezoid-shaped filter. As shown in FIG. 2C,lid 18 and base 10 defined a fluid egress region from the filter 16adjacent to the downstream edge of filter 20. A cross section of exitregion 17 immediately adjacent fluid egress region 16 had dimensionsapproximately 25 μm by 1.0 cm.

For the embodiment depicted in FIG. 2, a thin film lamination may beapplied to the filter to increase the filter's rigidity, to avoid havingthe filter fall into dead space 24 when sample is added. The laminationalso prevents bending of the filter in a region where the filter isunsupported; bending can lead to flow of fluid and particulates along aperipheral surface causing contamination of the filtrate, and isgenerally to be avoided. Pressure sensitive tape such as ARcare ®7396(Adhesives Research, Inc., Glen Rock, Pa.) is one example of acommercially available plastic lamination film which has been used.

The embodiment of FIG. 2 was used as follows: Fresh human whole blood(70 μl), was drawn in a Vacutainer® Blood Collection Tube with ACD, andwas added to fluid access port 14. Some of the fluid was immediatelyabsorbed into filter 20, and the remainder formed a small dropletcovering fluid access port 14. Due to the retarding of particulate flowthrough the filter, as the fluid laterally moved through filter 20, twodistinct flow fronts formed. The two flow fronts consisted of a clearplasma front preceding a dark front of particulate matter comprising redblood cells. Thus, the plasma front reached the downstream edge offilter 20 before the red cells, and by the time particulate matterreached the downstream edge, clear plasma had already entered exitcapillary 17. Exit capillary 17 contained material(s)/modality(s) forconducting an assay on the filtrate fluid, e.g., plasma, thus an assaywas performed substantially without contamination from particulatematter, e.g., red blood cells.

In general, fluid continues to flow from filter egress region 16 intoexit region 17 until the fluid sample at access port 14 is depleted intothe device or until the volume that exit region 17 can contain isfilled. With this concept in mind, an embodiment of a device of theinvention was designed to hold a specific volume of filtrate (e.g.,plasma) in exit region 17, for example, as in devices described in U.S.Pat. No. 5,458,852 to Buechler; once the exit region contained thespecified volume of liquid, fluid flow through the device stopped. Thefluid flow stopped whether or not sample remained at the access port 14or sample reservoir 26; thus, the addition of too much sample had nonegative affect on device function.

As depicted in Table 1, use of the embodiment of Example 2 yielded aflow rate of plasma out of filter 20 of up to 8 μl/min. Use of thisembodiment provided recovery of at least 6 μl of plasma. The effect ofvarying the area of filter compression at the distal compression region40 is shown in Table 1. As the area of the distal compression region 40was decreased from 0.041 in² to 0.004 in², the percent area ofcompression of the filter 20 concomitantly decreased from 32% to 3%, andthe flow rate of plasma through the filter increased from 1 μl/min to 8μl/min at a constant filter compression of 40%. This data showed thatthe flow rate of fluid from the filter can be controlled by the area ofthe filter that is compressed.

TABLE 1 EFFECT ON FLOW RATE BY VARIATIONS OF FILTER AREA COMPRESSED ATDISTAL REGION 40 (40% FILTER THICKNESS COMPRESSION) FLOW RATE AREA¹ %AREA COMP² μl/MIN 0.041 32 1 0.028 22 2 0.014 11 4 0.004  3 8 ¹Surfacearea of region 40 of structure 22 in square inches ²% of total filterarea that is compressed

Example Three

FIG. 3 represents an alternative embodiment of a filtration device inaccordance with this invention. The embodiment in FIG. 3 is fabricatedby conventional methodologies, such as described in Examples 1 and 2. Anaspect of this embodiment is the extension of compression structure 22along two additional sides (lateral compression regions 36) of base 10.Providing lateral compression regions 36 capable of compressing thefilter, diminished the possibility that fluid could create a lowresistance path along the peripheral surfaces of the filter, wherebyparticulate matter could contaminate the filtrate. In addition, thelateral compression regions of compression structure 22 providedadditional support for the filter 20, minimizing the potential for thefilter 20 to touch the bottom of filter cavity 12 in any aspect of deadspace 24.

Example Four

FIG. 4 represents an alternative embodiment of a filtration device inaccordance with the invention. This embodiment was fabricated inaccordance with standard methodologies, such as those described inExamples 1 and 2. An aspect of this embodiment is compression structure22 which comprises lateral compression regions 36 and proximalcompression region 38 so that the perimeter of filter 20 is supportedand capable of being compressed. A compression structure 22, as shown inFIG. 4A, is particularly useful when a large bolus of sample is added tothe device at a rapid rate. Thus, the extra support given along theedges of the filter will minimize the potential for the filter to touchthe bottom of the filter cavity 12 in any aspect of dead space 24.

The embodiment described in this example, and depicted in FIG. 4,provides a larger and more stable compression structure 22 for filter20. As previously described herein, dead space 24 is beneficial becauseit has essentially no capillary force, and therefore, dead space 24remains free of fluid throughout the operation of the embodimentdepicted in FIG. 4.

Depending on the manner of sample addition to the filtration device,that is, whether a large bolus of sample is added quickly to the filter,the degree of compression of the filter at the compression region can beincreased to about 50% of the filter thickness. This amount ofcompression, however, impacts the flow rate through the filter.Generally, as the filter is compressed more than about 5% of the filterthickness, the flow rate of the sample moving in the filter isdecreased.

As illustrated in FIGS. 4D and 4E, lid 18 incorporates a lid cavity 42which is positioned above filter 20 when lid 18 is assembled to base 10;the lid cavity does not extend into the area above the filter at thecompression structure 22. Thus, compression structure 22 is capable ofcompressing filter 20 to any degree necessary to seal and hold filter 20in filter cavity 12 without compressing the complete filter 20 againstthe lid 18, thereby potentially slowing overall fluid flow throughfilter 20.

Table 2 shows the effect of varying the areas of the proximal 38 andlateral 36 regions, wherein the area of distal compression region 40 washeld constant. The results showed that as the total areas were increasedfrom having no lateral or proximal compression region to a combined areaof 0.05 in², the flow rate decreased from 5 μl/min to 0.5 μl/min.

Thus, Table 2 provides data showing the impact on flow rate due tovariations in the combined area of proximal region 38 and lateral region36 of compression structure 22.

TABLE 2 EFFECT OF VARIATION OF COMBINED AREAS OF PROXIMAL REGION 38 ANDLATERAL REGION 36, ON FLUID FLOW RATE (AREA OF 40 WAS CONSTANT) FLOWRATE AREA¹ % AREA COMP² μl/MIN 0 0 5 0.01 8 2 0.025 20 1 0.05 39 0.5¹Combined area of proximal region 38 and lateral regions 36 of structure22 in square inches ²% of total filter area that is compressed

Example Five

FIG. 5 illustrates another embodiment of compression structure 22.Specifically, compression structure 22 was extended to comprise supportbar 23 (FIG. 5A, 5B), that divided dead space 24 into two separate deadspaces. Support bar 23 of compression structure 22 added additionalsupport to the filter in the area of the filter 20 which is adjacent andslightly downstream to fluid access port 14. In addition, support bar 23can provide for a differential compression as compared to other regionsof the compression structure 22. Support bar 23 can provide just enoughcompression on a small surface area of the filter such that flow ofsample between a peripheral filter surface and the cavity wall iseliminated, but not so much compression as to impede or cease fluid flowthrough the device.

Table 3 depicts data for flow rate based on variations in the area ofdistal compression region 40 of compression structure 22. Table 3provides empirical data that relates flow rate, plasma recovery volume,and k filter compression for the embodiment shown in FIGS. 5A-C. Thus,Table 3 illustrates that compressing filter 20 from 4% to 31% of thenative filter thickness at distal compression region 40, decreased theflow rate from about 3 μl/min to 2 μl/min while maintaining regionspecific compression. The data of Table 3 was taken by adding freshhuman whole blood to the inlet port of devices embodied as illustratedin FIG. 5. Each such device consisted of base 10 (Polysar, PolysarIncorporated, Madison, Conn., NAS® 30) ultrasonically bonded to lid 18with filter 20 (Ahlstrom CytoSep, Lot 94-49B, 0.066 cm thick) placedtherebetween.

TABLE 3 Flow rate of Distal Filter Plasma Exiting Plasma RecoveryCompression Filter Volume (%) (μl/min.) (μl)  4 3 6 15 3 6 23 2.5 8 31 28

Preferably, the surface area of the compression structure 22 thatcontacts filter 20 is held to a minimum. If one were to compress theentire filter, the flow rate and filtrate fluid recovery from filter 20would substantially decrease because the effective porosity of the totalfilter would be lowered.

In each of Examples 2-5, the sealing of filter 20 comprised use ofcompression in certain regions of filter 20. With each example, avariation in the shape of compression structure 22 was made. The choiceof a compression region configuration for use with a particulate sampleis appreciated readily by one of ordinary skill in the art in view ofthe present disclosure, based on the extent to which one wishes toemphasize parameters such as filter support, flow rate or particleretardation. For example, inclusion of compression structure 22 in FIG.3 reduced the leakage of red blood cells into the plasma and increasedseparation efficiency while maximizing flow rate.

Example 6

FIG. 6A-D illustrates a presently preferred embodiment of the invention.This embodiment provided a sample reservoir 26 (FIG. 6D) which safelycontains a fluid sample. Sample reservoir 26 preferably comprises acapillary space, and is in fluid communication with filter 20 and exitregion 17.

The embodiment depicted in FIG. 6 was fabricated as follows: base 10 wasfabricated using injection molding technology and was made of whiteacrylic copolymer (Polysar NAS® 30). The overall device length, widthand thickness were approximately 8.7, 3.5 and 0.26 cm, respectively. Thesurface of base 10 was made hydrophilic by treatment with a gas plasmain accordance with standard methodologies, alternatively coronadischarge treatment was used to make the surface hydrophilic.

As depicted in FIG. 6D, a slightly oversized filter 20 was fitted overcompression structure 22 and a dual region dead space 24. Filter 20(Ahlstrom CytoSep® filter, Mt. Holly Springs, Pa.) comprised a mixtureof cellulose, borosilicate glass, and polyester fibers. Its thicknesswas 0.063 cm and had an absorption volume of approximately 200 μl. Toenhance filter support, filter posts 28 were added, these posts arebelieved to improve the stability and positioning of filter 20.

Lid 18, formed of clear plastic, was ultrasonically welded to base 10.The lid had dimensions of approximately 8, 2.5 and 0.1 cm. Bonding lid18 to base 10 resulted in a slight compression of filter 20 ofapproximately 4% relative to its native thickness. Region-specificcompression at structure 22 created a seal that helped to prevent fluidand particle flow over peripheral surfaces of filter 20.

Tables 4-6 contain flow rate data for exit region 17. The data forTables 4-6 were obtained from experiments wherein at least 15 μl offiltrate fluid was obtained.

For example, as indicated in Table 4, a 25% compression of filter 20 bycompression structure 22 slowed the flow rate of the sample by as muchas 23% in the filter. Increasing the compression to about 50% decreasedthe flow rate by up to 61%.

TABLE 4 EFFECT ON FLUID FLOW BY FILTER COMPRESSION (THICKNESS) BY DISTALCOMPRESSION REGION 40 OF COMPRESSION STRUCTURE 22 % decrease % Flow Ratein fluid Compression μl/min flow  1 6.4 Control 25 4.9 23% 50 2.5 61%

Table 5 shows the effect of filter compression by support bar 23 on thefluid flow from the filter. A constant 1% compression of filter 20 atthe distal compression region 40 was the control. As the compression ofthe filter thickness by support bar 23 increased from 1% to 50%, theflow rate dropped from 6.4 μl/min to 1.7 μl/min.

TABLE 5 EFFECT ON FLUID FLOW BY FILTER COMPRESSION (THICKNESS) BYSUPPORT BAR 23 % decrease % Flow Rate in fluid Compression μl/min flow 1 6.4 Control 25 3.0 53% 50 1.7 73%

Table 6 shows the effect of filter compression at both distal region 40and support bar 23, on fluid flow rate from the filter. As thecompression regions increased filter compression from 1% to 50%, theflow rate decreased from 6.4 μl/min to 1.8 μl/min.

TABLE 6 EFFECT ON FLUID FLOW BY FILTER COMPRESSION (THICKNESS) BY DISTALREGION 40 AND SUPPORT BAR 23 % decrease % Flow Rate in fluid Compressionμl/min flow  1 6.4 Control 25 2.9 55% 50 1.8 72%

The sample flow rate decreased since pores or matrices of the filter aremade smaller by compression and the resistance to flow is increased. Asdisclosed herein, compression of the filter at compression structure 22can be as great as 50% if lid 18 has been modified to comprise lidcavity 42, as shown in FIG. 4D, without a substantial decrease in fluidflow rate.

Fluid access port 14 was located over sample reservoir 26. One skilledin the art will recognize that the fluid access port 14 can be situatedover the sample reservoir 26, as well as a portion of filter 20 (FIG.6D). Lid 18 and base 10 formed a fluid exit region 17 (FIG. 6D) adjacentto the downstream edge of filter 20 (i.e., adjacent fluid egress region16 of filter 20). The fluid exit region preferably can comprise acapillary space. In an embodiment where exit region 17 comprised acapillary, exit region 17 had cross-sectional dimensions adjacent filteregress region 16 of 25 μm by 1.0 cm to create a capillary gap (alsoreferred to herein as a capillary space). The dimension of a fluidegress region comprising a capillary can vary from about 0.1 μm to about100 μm, and most preferably from about 10 μm to 50 μm.

The capillarity of exit region 17 was important for embodiments wherefluid was drawn into the exit region without an external appliedpressure at any point before or along the flow path. For suchembodiments, the relative capillarity of filter 20 and the exit region17 were designed to provide fluid movement from the filter into the exitcapillary. Thus, exit region 17 will have the highest capillarity,filter 20 will have intermediate capillarity and sample reservoir 26will have lowest capillarity. The capillarity of the sample reservoirshould not be so great as to prevent fluid from entering the filter andthe downstream capillaries of the device, particularly devices asdescribed in U.S. Pat. No. 5,458,852.

As depicted in FIG. 6D, sample reservoir 26, is in fluid communicationwith the top, bottom and edge of filter 20. One skilled in the art willrecognize that the surface area of the filter in contact with fluid insample reservoir 26 can be varied, and that maximizing the surface areaof the filter in contact with fluid will improve the filtrationcharacteristics of the filter because particulate matter incapable ofpenetrating the filter is spread out over a larger area, whereby thefilter will have a lower tendency to clog.

Sample reservoir 26 also facilitates addition of various volumes ofsample to the device. One skilled in the art will recognize that thevolume of the sample reservoir 26 should be adjusted to accommodate thevolume of sample to be assayed by the device. Preferred volumes for thesample reservoir are between 0.1 μl and 1000 μl, and particularlypreferred volumes are between 5 μl and 300 μl.

In one embodiment, sample reservoir 26 comprises a capillary space whena lid 18 is attached to the base 10; therefore, the dimensions of thecapillary gap of the reservoir 26 are designed in accordance with thecapillarity of the regions of the entire device, so that there will befluid flow and filtration.

Advantageously, the capillary force of the fluid reservoir 26 held thefluid sample within the reservoir and minimized the risk of spilling orexpulsion of fluid sample from the device. This is particularlyadvantageous when the fluid sample is, e.g., a biological fluid that maybe contaminated with hazardous bacteria or virus, or is an environmentalwater sample contaminated with pollutants.

Preferably, reservoir 26 comprises vent holes 30 in lid 18. Vent holes30 allow escape of air from reservoir 28 during filling with the fluidsample, and facilitate more complete filling of the reservoir.

A particularly preferred embodiment of the sample reservoir 26, asillustrated in FIG. 6, comprises grooves 34 on the floor of reservoir 26and on the filter stay 32. In the absence of grooves (or other suitabletexture), when the floor of the sample reservoir 26 was flat, somesample fluid was retained by capillary action in the corners of thesample reservoir 26. In one embodiment, grooves 34 are approximately0.013 cm deep and 0.043 cm wide and are oriented such that the samplefluid is directed to filter 20 by flowing along grooves 34. Thus, thesample fluid completely drained from the corners of the sample reservoir26, since grooves 34 served to break the capillary tension of the samplefluid meniscus that formed in the corners of sample reservoir 26 thatoccurred as sample fluid was depleted from the reservoir.

One skilled in the art will recognize that the size and orientation ofthe grooves can be changed and the grooves could be substituted for asurface texture of posts protruding from the floor of the samplereservoir without changing the scope of this invention.

In the particular embodiment with dimensions discussed in this example,fresh human whole blood (220 μl), drawn in a Vacutainer® BloodCollection Tube with EDTA, was added through fluid access port 14, intosample reservoir 26; the fluid simultaneously began to flow into filter20. Blood continued to move from sample reservoir 26 into filter 20because filter 20 had a stronger capillary force than sample reservoir26. The fluid flowed laterally through filter 20, and two distinct flowfronts developed, the leading front being clear plasma and the trailingfront comprising particulate material such as red blood cells. Theplasma front reached the downstream edge of filter 20 (i.e., fluidegress region 16) after about 50 seconds at which point the plasma frontwas approximately 2 mm ahead of the red blood cell front. Within filter20 having dimensions disclosed above, the volume of plasma that a 2 mmdistance corresponded to was approximately 20 to 30 μl. This volumerepresented the amount of plasma available for an assay before the redblood cells reached the downstream edge of filter 20 and could move intoexit region 17; this configuration was selected because a volume of 15to 20 μl was necessary to obtain an assay result. Fluid flow throughfilter 20 was enhanced by the capillary force of exit region 17. Asnoted above, exit region 17 comprised a capillary space, and plasmaexited the filter and entered exit region 17 by capillary action, at aflow rate of approximately 7 μl/min. Approximately 15 to 20 μl of plasmawas recovered and was devoid of red blood cells.

In a preferred embodiment, to minimize the possibility that a filtratefluid in exit region 17 might be contaminated with particulate matter,the exit region was designed to accommodate a volume which correspondedto the volume of fluid that was equal to or less than that whichprecedes the particulate matter front for a given device embodiment.Thus, in the embodiment depicted in FIG. 6, with dimensions as set forthabove, the exit region was designed to accommodate 20-30 μl or less.

1. A device comprising: a filter capable of separating particulatematter from liquid matter in a fluid sample, and a means for regionspecific compression of the filter, wherein said filter is composed of asingle filter layer and said filter is not compressed in all regions. 2.The device of claim 1, whereby the means for region specific compressionretards particulate matter in a fluid sample from moving from the filterand into a capillary space external to the filter and adjacent to wherethe filter is sealed.
 3. The device of claim 1 further comprising afluid impermeable glue, a sealant, or a pressure adhesive tape toprevent particulate escape and the flow of liquid along peripheralsurfaces of the filter.
 4. The device of claim 1, wherein the means forregion specific compression comprises compressing the filter between twoor more solid surfaces.
 5. The device of claim 4, wherein the regionspecific compression comprises compression of the filter by 1-50% of thenative thickness of the filter.
 6. The device of claim 1, furthercomprising a capillary means in fluid communication with said filter. 7.The device of claim 1, further comprising a means for producing an assayresponse for an analyte of interest.
 8. The device of claim 1, whereinthe filter is selected to provide separation of red blood cells fromplasma.